GB689285A - Improvements in and relating to the catalytic cracking of hydrocarbon oils - Google Patents

Abstract

<PICT:0689285/III/1> <PICT:0689285/III/2> The hydrocarbon oil is divided into two fractions; the higher-boiling fraction is cracked in the presence of finely-divided catalyst in an upflow reactor under conditions of hindered settling, and the lower-boiling fraction is cracked in the presence of catalyst which has already been used to crack the higher-boiling fraction, the catalyst in this stage being maintained as a dense, fluidized, stationary bed in the lower part of a second reactor, which has a bottom draw-off leading to a regeneration zone. This two-fold use of catalyst is achieved by passing the suspension withdrawn from the top of the upflow reactor into the disengaging zone above the dense bed in the downflow reactor. The products from both cracking reactions are withdrawn together from the top of the downflow reactor. The residence time of the catalyst, in the upper sector is preferably relatively short compared with its residence time in the downflow reactor. The temperature may be same in both reactors, or the temperature in the upflow reactor may be higher than that in the downflow reactor. A reduced crude petroleum is passed into unit 1, Fig. I, where it is vaporized, and freed from asphalt and salts. The vapours are fractionated in column 3. The overhead fraction has an average molecular weight of about 300, and amounts to about 75 per cent of the feed. The bottom fraction, which has an average molecular weight of 660, is pumped through line 5, and is re-vaporized by hot freshly regenerated catalyst, introduced through line 6, the proportion of hot catalyst being sufficient not only to vaporize the fraction but to supply the necessary heat for cracking to a depth of about 30 to 70 per cent. The mixture of catalyst and vaporized oil passes via lines 8 and 81 to two upflow reactors 9 and 91, wherein the temperature is maintained at 496 DEG C, the average catalyst density at 528 g./litre, and the weight hourly space velocity at 2.8. The suspension of catalyst in the vapours of the cracked products and of uncracked oil, is passed through lines 10 and 101 into downflow reactor 11. Lines 10 and 101 enter the disengaging zone of reactor 11 tangentially, and the suspension is then deflected by baffle 12 (Fig. II), downwards towards the dense bed of catalyst which accumulates in the lower part of reactor 11. The overhead fraction from column 3 is condensed, and the condensate is pumped through line 14 into the base of reactor 11. In some cases it may be desirable to supply, via lines 22 and 14, a portion of freshly regenerated catalyst directly to reactor 11 p together with the lower boiling fraction. The temperature in the dense bed is maintained at 496 DEG C, the catalyst density at 640 g./litre, and the weight hourly space velocity at 0.8. A mixture of cracked vapours and vapours of uncracked oil is withdrawn from the top of reactor 11 via cyclone separators 16 and line 15; and the mixture is fractionated in column 18. A slurry of fine-catalyst-in-heavy-oil is withdrawn from base of column 18, and may if desired be passed to vaporizing unit 1. A portion of the spent catalyst is continuously withdrawn from the base of reactor 11 through a baffled annular space, countercurrent to a stream of steam or other stripping gas introduced via lines 21 and 211. The stripped catalyst passes down standpipes 19 and 191, and is then passed, by a stream of air, through line 20 to regeneration zone 7. The upflow reactors may alternatively be constructed inside the downflow reactor.